TableTransforms.jl
Transforms and pipelines with tabular data.
Overview
This package provides transforms that are commonly used in statistics and machine learning. It was developed to address specific needs in feature engineering and works with general Tables.jl tables.
Past attempts to model transforms in Julia such as FeatureTransforms.jl served as inspiration for this package. We are happy to absorb any missing transform, and contributions are very welcome.
Features
Transforms are revertible meaning that one can apply a transform and undo the transformation without having to do all the manual work keeping constants around.
Pipelines can be easily constructed with clean syntax
(f1 → f2 → f3) ⊔ (f4 → f5)
, and they are automatically revertible when the individual transforms are revertible.Branches of a pipeline and colwise transforms are run in parallel using multiple processes with the Distributed standard library.
Pipelines can be reapplied to unseen "test" data using the same cache (e.g. constants) fitted with "training" data. For example, a
ZScore
relies on "fitting"μ
andσ
once at training time.
Rationale
A common task in statistics and machine learning consists of transforming the variables of a problem to achieve better convergence or to apply methods that rely on multivariate Gaussian distributions. This process can be quite tedious to implement by hand and very error-prone. We provide a consistent and clean API to combine statistical transforms into pipelines.
Although most transforms discussed here come from the statistical domain, our long term vision is more ambitious. We aim to provide a complete user experience with fully-featured pipelines that include standardization of column names, imputation of missing data, and more.
Usage
Consider the following table and its pairplot:
using TableTransforms
using CairoMakie, PairPlots
# example table from PairPlots.jl
N = 100_000
a = [2randn(N÷2) .+ 6; randn(N÷2)]
b = [3randn(N÷2); 2randn(N÷2)]
c = randn(N)
d = c .+ 0.6randn(N)
table = (; a, b, c, d)
# pairplot of original table
table |> pairplot
We can convert the columns to PCA scores:
# convert to PCA scores
table |> PCA() |> pairplot
or to any marginal distribution:
using Distributions
# convert to any Distributions.jl
table |> Quantile(dist=Normal()) |> pairplot
Below is a more sophisticated example with a pipeline that has two parallel branches. The tables produced by these two branches are concatenated horizontally in the final table:
# create a transform pipeline
f1 = ZScore()
f2 = LowHigh()
f3 = Quantile()
f4 = Functional(cos)
f5 = Interquartile()
pipeline = (f1 → f2 → f3) ⊔ (f4 → f5)
# feed data into the pipeline
table |> pipeline |> pairplot
Each branch is a sequence of transforms constructed with the →
(\to<tab>
) operator. The branches are placed in parallel with the ⊔
(\sqcup<tab>
) operator.
TransformsBase.:→
— Functiontransform₁ → transform₂ → ⋯ → transformₙ
Create a SequentialTransform
transform with [transform₁, transform₂, …, transformₙ]
.
TransformsBase.SequentialTransform
— TypeSequentialTransform(transforms)
A transform where transforms
are applied in sequence.
TableTransforms.:⊔
— Functiontransform₁ ⊔ transform₂ ⊔ ⋯ ⊔ transformₙ
Create a ParallelTableTransform
transform with [transform₁, transform₂, …, transformₙ]
.
TableTransforms.ParallelTableTransform
— TypeParallelTableTransform(transforms)
A transform where transforms
are applied in parallel. It isrevertible
if any of the constituent transforms
is revertible. In this case, the revert
is performed with the first revertible transform in the list.
Examples
LowHigh(low=0.3, high=0.6) ⊔ EigenAnalysis(:VDV)
ZScore() ⊔ EigenAnalysis(:V)
Notes
- Metadata is transformed with the first revertible transform in the list of
transforms
.
Reverting transforms
To revert a pipeline or single transform, use the apply
and revert
functions instead. The function isrevertible
can be used to check if a transform is revertible.
TransformsBase.apply
— Functionnewobject, cache = apply(transform, object)
Apply transform
on the object
. Return the newobject
and a cache
to revert the transform later.
TransformsBase.revert
— Functionobject = revert(transform, newobject, cache)
Revert the transform
on the newobject
using the cache
from the corresponding apply
call and return the original object
. Only defined when the transform
isrevertible
.
TransformsBase.isrevertible
— Functionisrevertible(transform)
Tells whether or not the transform
is revertible, i.e. supports a revert
function. Defaults to false
for new transform types.
Transforms can be revertible and yet don't be invertible. Invertibility is a mathematical concept, whereas revertibility is a computational concept.
See also isinvertible
.
To exemplify the use of these functions, let's create a table:
a = [-1.0, 4.0, 1.6, 3.4]
b = [1.6, 3.4, -1.0, 4.0]
c = [3.4, 2.0, 3.6, -1.0]
table = (; a, b, c)
(a = [-1.0, 4.0, 1.6, 3.4], b = [1.6, 3.4, -1.0, 4.0], c = [3.4, 2.0, 3.6, -1.0])
Now, let's choose a transform and check that it is revertible:
transform = Center()
isrevertible(transform)
true
We apply the transformation to the table and save the cache in a variable:
newtable, cache = apply(transform, table)
newtable
(a = [-3.0, 2.0, -0.3999999999999999, 1.4], b = [-0.3999999999999999, 1.4, -3.0, 2.0], c = [1.4, 0.0, 1.6, -3.0])
Using the cache we can revert the transform:
original = revert(transform, newtable, cache)
(a = [-1.0, 4.0, 1.6, 3.4], b = [1.6, 3.4, -1.0, 4.0], c = [3.4, 2.0, 3.6, -1.0])
Inverting transforms
Some transforms have an inverse that can be created with the inverse
function. The function isinvertible
can be used to check if a transform is invertible.
TransformsBase.inverse
— FunctionTransformsBase.isinvertible
— Functionisinvertible(transform)
Tells whether or not the transform
is invertible, i.e. whether it implements the inverse
function. Defaults to false
for new transform types.
Transforms can be invertible in the mathematical sense, i.e., there exists a one-to-one mapping between input and output spaces.
See also inverse
, isrevertible
.
Let's exemplify this:
a = [5.1, 1.5, 9.4, 2.4]
b = [7.6, 6.2, 5.8, 3.0]
c = [6.3, 7.9, 7.6, 8.4]
table = (; a, b, c)
(a = [5.1, 1.5, 9.4, 2.4], b = [7.6, 6.2, 5.8, 3.0], c = [6.3, 7.9, 7.6, 8.4])
Choose a transform and check that it is invertible:
transform = Functional(exp)
isinvertible(transform)
true
Now, let's test the inverse transform:
invtransform = inverse(transform)
invtransform(transform(table))
(a = [5.1, 1.5, 9.4, 2.4], b = [7.6, 6.2, 5.8, 3.0], c = [6.3, 7.9, 7.6, 8.4])
Reapplying transforms
Finally, it is sometimes useful to reapply
a transform that was "fitted" with training data to unseen test data. In this case, the cache from a previous apply
call is used:
TransformsBase.reapply
— Functionnewobject = reapply(transform, object, cache)
Reapply the transform
to (a possibly different) object
using a cache
that was created with a previous apply
call. Fallback to apply
without using the cache
.
Consider the following example:
traintable = (a = rand(3), b = rand(3), c = rand(3))
testtable = (a = rand(3), b = rand(3), c = rand(3))
transform = ZScore()
# ZScore transform "fits" μ and σ using training data
newtable, cache = apply(transform, traintable)
# we can reuse the same values of μ and σ with test data
newtable = reapply(transform, testtable, cache)
(a = [2.34346832035787, 2.4470491961453615, -0.022640928105979813], b = [2.689767970862668, 10.866916251843252, 9.82240947156653], c = [0.8881917706454474, 1.498434223107, -0.6105760941694538])
Note that this result is different from the result returned by the apply
function:
newtable, cache = apply(transform, testtable)
newtable
(a = [0.539879021875904, 0.614027672448252, -1.1539066943241552], b = [-1.146721965172694, 0.6907132341320911, 0.45600873104060374], c = [0.27290920626143433, 0.8352142827832635, -1.108123489044697])